What does the 'Dim' switch do on my legacy Saffire?

This is a feature that is common on large consoles in studios, it reduces the output level of the monitors (any outputs with the 'H' button active in SaffireControl - normally just o/ps 1 and 2), in this case by 12dB. This gives you more control over a highly sensitive pair of connected speakers. Explained more simply: Imagine the gain dial is turned down really low on the amp/speakers connected to the Saffire and the music is still coming out pretty loud and just turning up the dial a little bit makes it significantly louder. The solution to this is to press the DIM button on the Saffire, then the volume of the music will go down (by 12dB) and you can turn up that dial and have a much better control over the volume.